This application is a National Stage filing under 35 U.S.C. xc2xa7371 of International Application No. PCT/CH00/00210 having an International Filing Date of Apr. 11, 2000, the entire contents of which are incorporated herein in its entirety.
The invention is based on an electrical insulator for supporting an electrode, which carries high voltage, on a holder as claimed in the precharacterizing clause of patent claim 1.
Such an insulator is used, for example, in medium-voltage and high-voltage technology for supporting a live electrical conductor with respect to grounded parts of a system.
An electrical insulator of the type mentioned initially is described, for example in DE 40 07 337 A1. In this insulator, surfaces of an insulated body are coated with an insulating glazing compound with a high dielectric constant for field control.
The surface of the insulating body is a dielectric weak point of the insulator. Electrically conductive particles, such a metal swarf, which enters the encapsulation during installation, or particles detached from the conductor, which can be formed by thermal cycling and the movement resulting from it, have the tendency to adhere to the surface of the insulator body, owing to the high dielectric constant of said insulator body. Such particles lead to a considerably reduced breakdown voltage, since the electric field around the particles is increased by the proximity of the insulator body, resulting in a field peak. A critical zone with regard to flashovers is formed in the region of this field peak.
An electrode for controlling electric fields in a gas-insulated area is described in WO 98/22958. This electrode has an electrically conductive surface and is at least partially coated with a material which has a low dielectric constant. The material contains, for example, a porous foam which, thanks to the enclosed gas, has approximately the same dielectric constant as the surrounding gas, but with a dielectric strength which is greater than that of that gas.
A coating of an insulating supporting element for electrodes, which are directly seated or arranged at a certain distance, for high-voltage switchgear assemblies is known from DE 3140652. This coating is composed of a material having a dielectric constant which is less than that of the material of the supporting element.
The invention, as it is specified in patent claim 1, is based on the object of providing an electrical insulator of the type mentioned initially which, despite having a compact construction, is distinguished by high operational reliability when the dielectric load is high.
The surface of the insulator with an insulator body is at least partially formed from a protective body. This has a structure whose mean dielectric constant is less than that of the material of the insulator body. This makes it possible to avoid an excessive field peak in the region of a conductive particle located on the surface of the insulator. In order to achieve a field peak which is as low as possible, the protective body should have a thickness which corresponds to the maximum length of the particles to be considered.
In a first embodiment, the protective body contains a foam. In contrast to the coatings for insulator bodies which are known from the prior art, a gaseous medium surrounding the insulator penetrates into the pores of the foam, thus resulting in the foam having a mean dielectric constant which virtually corresponds to that of the gaseous medium. Syntactic foams are particularly advantageous, since the size of the pores can be controlled well, and this is of major importance for the dielectric strength of the material.
In a further embodiment, the protective body is in the form of a honeycomb, or is formed from a large number of thin walls arranged parallel to the lines of force. The thin walls and the small contact surface areas associated with them advantageously result in small critical zones.
If the insulator is attached to a grounded holder and/or supports an electrical conductor, then it is particularly advantageous to use a flexible or elastic foam. This is guided along the conductor and/or the grounded holder, and thus prevents the ingress of particles into any gap which may be present in the region of the triple point between the insulator body and the holder.
A gas-insulated system having such insulators can be produced more compactly and more cheaply and can be designed for higher voltages while, at the same time, having a longer life expectancy.